Monolateral external fixator (MEF) has been commonly used for limb lengthening in both upper and lower extremities. Since MEF can only lengthen or shorten the bone, sagittal, coronal, and torsional deformities need to be corrected acutely at the time of MEF application. Acute correction, particularly torsional correction, may endanger the radial or peroneal nerves. Gradual correction of torsion may be safely achieved using a circular external fixator (CEF) frame. We present a case study of humeral lengthening with derotation to describe a technique, whereby it is possible to first lengthen with MEF and then derotate with CEF. This is a case report of a 14-year-old boy with a history of neonatal sepsis and growth arrest of the proximal humerus that led to a short right humerus and loss of external rotation. Humerus lengthening was achieved (6.6 cm) using MEF. Conversion to CEF was done after the distraction phase and prior to regenerate bone consolidation. Two 2/3 rings were connected to the proximal and distal fixation half-pins, and then the six connecting struts were added. The rotation deformity correction was then performed gradually to achieve 60° of external rotation. The described technique to convert the MEF into CEF is versatile and can be employed in the clinic safely and without sedation.

Distraction osteogenesis with external fixators is commonly done for bone lengthening. Choice of fixation includes monolateral external fixators (MEFs) and circular external fixators (CEFs).[1],[2] MEFs are less bulky compared to the circular fixators.[3],[4],[5],[6] Although MEFs have a sleeker profile and are preferred by patients, they are limited to lengthening and shortening.[7] CEFs are bulky but are capable of gradual correction of angular and torsional deformities.[4],[5] We present in this study a case of 6.5-cm humerus lengthening that was done using a low-profile MEF modified to a CEF at a second stage for rotation correction. The fixator conversion was performed in the clinic without sedation.

Case Report

A 14-year-old boy presented to the orthopedic clinic at the Rubin Institute for Advanced Orthopedics, Sinai Hospital of Baltimore, with a short right arm which resulted from proximal humeral growth arrest caused by neonatal septic osteoarthritis/arthritis. The main complaints were short arm and lack of shoulder range of motion. On physical examination, the right arm had 70° of abduction and 10° of extension compared to the normal left arm. The radiographs showed 5.7-cm length difference with an estimated 8-cm difference at skeletal maturity [Figure 1]. The treatment plan was to perform straight lengthening of the right arm using a MEF (LRS, Orthofix, Lewisville, TX), and then to modify the fixator to a CEF (Taylor Spatial Frame, Smith & Nephew, Memphis, TN) for derotation adjustment.

Figure 1: Right humerus radiographs before surgery (a), immediate postoperative (b), and at the end of lengthening (c)

The MEF construct was built of three pins proximally and two pins distally. The osteotomy was done at the level of the deltoid tuberosity. The distraction phase (0.25 mm three times a day) lasted for 4 months and achieved 6.6 cm of length. At that point, radiographs showed appropriate regenerate bone that was not consolidated yet. The MEF frame was converted to CEF to address the rotational deformity.

The MEF was converted into CEF in the clinic with no need for sedation, local anesthesia, or fluoroscopy. Given that the regenerate bone was not consolidated, careful attention was paid to maintain stability of the fixation to avoid displacement of the delicate regenerate bone during the conversion process.

Step 1: Apply another MEF (same size as the original) so that it is piggybacked over the existing pins

Step 2: Loosen the pin clamps on the original MEF, shift the MEF toward the skin as close as possible, and then retighten the clamps [Figure 2]a

Step 3: Remove the secondary (new) MEF. The purpose of this step is to increase the available length of the pins to allow application of the CEF [Figure 2]b. This process (steps 1–3) maintains the length of the regenerate bone

Step 4: Apply two 2/3 rings using Rancho cubes to connect the rings to the two-pin cluster proximally and distally. Direct the proximal ring opening toward the body and direct the distal ring opening anteriorly to allow elbow flexion [Figure 2]c. In this particular case, there was a third pin proximally that was inserted out of plane of the two-pin cluster. This third proximal pin was connected to the proximal ring by a 3-hole Rancho cube and a plate [Figure 2]d

Step 5: After the two rings are secured to the pins, the six struts are attached to the rings [Figure 2]e. Strut lengths and sizes are documented for input into the program setting

Step 6: A 5-hole cube and a socket are applied to mark the anteroposterior and lateral radiographs for mounting parameters [Figure 2]f

Step 7: Finally, the original MEF is removed after stabilizing the humerus with the CEF [Figure 2]g.

Figure 2: Clinical photographs demonstrating the steps of monolateral external fixator conversion to circular external fixator. (a) Apply another monolateral external fixator that is the same size over the existing pins. (b) Remove the secondary (new) monolateral external fixator. (c) Apply two 2/3 rings using Rancho cubes to connect the rings to the two-pin cluster proximally and distally. (d) The third proximal pin is connected by a 3-hole Rancho cube and a plate. (e) The rings are secured to the pins and connected with six struts. (f) A 5-hole cube and a socket are applied to mark the anteroposterior and lateral radiographs for mounting parameters. (g) The original monolateral external fixator is removed

The CEF strut turning schedule was then programmed (deformity parameter was 25° internal rotation). With a correction rate of 0.5 mm per day, the turning schedule lasted for 18 days. A residual deformity of 11° of varus and mild translation was corrected using a secondary residual correction schedule. The fixator was then maintained for 4 months to achieve full regenerate bone consolidation [Figure 3]. At the last follow-up, the patient was able to achieve functional range of shoulder external rotation (60°) [Figure 4].

MEF provides advantages over CEF, especially in the upper extremity. First, MEF is less bulky, less complicated, and causes less discomfort compared to CEF.[5],[8] Second, MEF minimally blocks shoulder or elbow movement compared to CEF.[3],[6] Pins in MEF are located on one plane (coronal plane) compared with CEF where the multiplanar wires or pin fixation may increase the chance of interference with muscle excursion and theoretically increase the chance of neurovascular injury.[6] Furthermore, multiplanar struts of the CEF can overlap the regenerate bone on radiographs and hinder regenerate bone assessment compared to MEF where shadows from the frame can more easily be avoided.

The lengthening rate in the humerus is generally 0.75–1 mm per day starting from postoperative day 6 or 7. The lengthening rate may need to be adjusted in response to common problems and obstacles (i.e., pin-site infection, pain, and limited range of motion).[9] Practically, modifying the lengthening rate is easier with MEF compared with CEF. Only a simple change of the number of turns per day is needed with MEF, whereas in CEF, a new strut turning schedule needs to be planned every time a change is needed.

The downside of using MEF is the lack of ability to achieve simultaneous correction of all deformities in all planes. Coronal and sagittal deformities need to be addressed at the time of fixator application. When CEF is used, all the deformities in all planes can be corrected simultaneously and gradually. With large rotational deformity, the CEF correction schedule is planned to address the lengthening and angulation deformities followed by derotation. This is analogous to wringing out a wet dish towel; if done over a large distance, less water (“blood”) is squeezed out (“vessel strangulation”) than if done over a short distance. The downside of CEF is size and possible interference with elbow and shoulder range of motion.

Notice that the derotation correction of the humerus caused a residual deformity of 11° of varus and mild lateral translation. This residual deformity was anticipated since the first CEF schedule was programmed with only one parameter (internal rotation). Therefore, mild varus and translation deformity was radiographically seen (not clinically significant) and corrected with a secondary residual correction program. Finally, the humerus osteotomy was performed proximally at the level of the deltoid tuberosity proximal to the medial head of the triceps.[10] Yasui et al. have described a technique to perform the humeral osteotomy distally above the olecranon fossa close to the deformity apex in patients with Apert syndrome.[11] Proximal humerous osteotomy reduces the risk of elbow flexion contracture compared with distal osteotomies.

We describe in this study a case of staged humerus lengthening using MEF followed by CEF. The MEF was converted to CEF once the lengthening was achieved but before regenerate bone consolidation. The fixator conversion technique is not complicated and can be performed in a clinic setting without sedation. This technique is versatile and can be employed in any condition that requires a MEF that needs to be converted to a CEF.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.